In the field of conveyor systems, moving chains or cables are utilized as a drive system for pulling conveyor carts or pallets through a number of conveyor stations where various manufacturing or other process operations are performed. However, chain or cable-driven systems generally must move along at a constant rate and the conveyor speed is therefore limited to the slowest rate utilized in the system. To overcome this problem, established independent control over the pallet can be utilized to accelerate the pallet into and out of each station, thereby reducing conveyor time.
Independent pallet control can be achieved through utilization of a linear motor driven system, thereby avoiding moving cables or chains. In a standard linear motor, an arrangement of stationary electromagnetic coil assemblies are configured to interact with a magnet assembly attached to a linear stage, e.g. the underside of a moving conveyor pallet. When the coil assemblies are energized, a magnetic force is imparted to the magnet assembly, which displaces the linear stage, thereby moving conveyor pallet. By controlling the application of current to the coils, each pallet can be controlled individually, with an infinite range of independent control over the speed and position of the pallet.
In a typical linear motor application, infinite independent control can be obtained by using a feedback system to sense the position of the stage, and in response to the position, commutating the independent stationary coils (e.g., turning the coils on and off in a coordinated control scheme) in order to control the speed and position of each pallet. This type of control scheme can give infinite control of the stage over the entire length of travel. However, this results in a very complex and difficult control scheme.
For example, a relatively small linear motor conveyor system having a twenty-five foot oval track would require twenty-five feet of coils. Each coil is about three inches in diameter, thus, a twenty-five foot section utilizes about one hundred coils. In order to establish independent control over four pallets moving through the conveyor, it would be necessary to sense the position of each pallet at all times and commutate the proper coils at the right times to obtain the desired motion and speed. Over a twenty-five foot curved path of travel, numerous sensors would be needed and switching devices would be required for each of the one hundred independent coils. A very complex control scheme would be necessary to implement such a system and would be inherently less reliable and more expensive because of the number of switching devices needed to control every independent coil.
While the benefits of linear motor conveyor systems are evident, the expense, reliability and maintenance issues associated with these systems have prevented their wholesale penetration into the conveyor market. In comparison, conventional chains and cables are very inexpensive, about one dollar per foot. So while many businesses that use conveyor systems would like to obtain the benefits of a linear motor drive, the expense and reliability factors have made conversion undesirable and in some cases prohibitive.